Bismuth-doped La1.75Sr0.25NiO4+δ as a novel cathode material for solid oxide fuel cells

Abstract

Bismuth has been doped into mixed ionic–electronic conducting La_1.75Sr_0.25NiO_4+δ (LSN) with the 2D K₂NiF₄-type structure to evaluate its influence on various properties of the host material, which include its potential use as a SOFC cathode. X-ray powder diffraction indicates that LSN retains its tetragonal structure after doping with 5 mol% bismuth to form La_1.65Bi_0.1Sr_0.25NiO_4+δ (LSN–Bi). Bismuth doping profoundly lowers (by ∼150 °C) the sintering temperature of LSN. Both LSN and LSN–Bi show excellent compatibility with electrolytes yttria-stabilized zirconia (YSZ) and samaria-doped ceria (SDC) in terms of thermal expansion and chemical reactivity (<900 °C). The electrical conductivity of both materials is metallic like and reaches values of 99.3 S cm⁻¹ and 100.3 S cm⁻¹ at 550 °C for LSN and LSN–Bi, respectively. The data from electrical conductivity relaxation (ECR) measurements demonstrate that the substitution of lanthanum by bismuth enhances the chemical diffusion coefficient (D_chem) and surface exchange coefficient (k_chem) by factors of 2–3. The faster kinetics of oxygen transport exhibited by LSN–Bi relative to parent LSN is reflected by a lower polarization resistance of the former when the electrode performance of both materials is compared in symmetric cells. The corresponding values at 700 °C are 4.2 Ω cm² and 0.61 Ω cm² for LSN and LSN–Bi, respectively. High peak power densities are achieved (328 mW cm⁻² and 131 mW cm⁻² at 700 and 600 °C, respectively), when LSN–Bi is incorporated as the cathode in a fuel cell operated with humidified hydrogen as the fuel and air as the cathode gas. The material is considered a promising candidate for further study.